The Problem With "220 Minus Your Age"
The most common formula for estimating maximum heart rate is:
Max HR = 220 – Age
It's simple, fast, and widely used. It's also quite inaccurate for a large portion of the population. Research shows that actual maximum heart rates vary by ±10–20 beats per minute from this estimate at any given age. That kind of error shifts all your zone boundaries significantly.
If your zones are off, you may think you're training in Zone 2 when you're actually in Zone 3 — or you might be sandbagging your threshold sessions. Either way, your training becomes less effective than it could be.
Enter the Karvonen Formula
The Karvonen formula, developed by Finnish scientist Martti Karvonen in the 1950s, introduces a critical variable that the simple formula ignores: your resting heart rate.
The formula works on Heart Rate Reserve (HRR) — the range between your resting heart rate and your maximum heart rate. This range is a much better representation of your actual cardiovascular capacity.
Heart Rate Reserve = Max HR – Resting HR
Then, each zone is calculated as:
Target HR = Resting HR + (HRR × Zone Intensity %)
A Real-World Example
Let's compare two 35-year-old runners:
- Runner A: Resting HR of 45 bpm (very fit)
- Runner B: Resting HR of 72 bpm (sedentary baseline)
Both have an estimated max HR of 185 bpm (220 – 35).
Zone 2 lower boundary (60%):
- Runner A: 45 + (140 × 0.60) = 129 bpm
- Runner B: 72 + (113 × 0.60) = 140 bpm
That's an 11 bpm difference. If Runner A trained using Runner B's zones, they'd be consistently under-training. If Runner B used Runner A's zones, they'd be over-training and burning out aerobic base sessions.
Why Resting Heart Rate Reflects Your Fitness
Your resting heart rate is a direct measure of how efficiently your heart pumps blood. A trained cardiovascular system moves more blood per beat, so it doesn't need to beat as often at rest. Elite endurance athletes often have resting heart rates in the 30–45 bpm range.
As your fitness improves over months of training, your resting heart rate will typically decrease. This means your Karvonen-based zones shift slightly — your Zone 2 ceiling will move up, reflecting real gains in aerobic capacity.
How ZoneSteady Uses This
When you set up ZoneSteady, you enter your age and resting heart rate. The app calculates your Heart Rate Reserve and applies the Karvonen formula to produce your five personalized zones.
During a workout, your Apple Watch streams your live heart rate to ZoneSteady, which maps it against your custom boundaries in real time. You see your current zone — not a generic estimate, but one built around your actual physiology.
To measure your resting heart rate accurately, check it first thing in the morning before getting out of bed. Take several readings over a few days and average them for the best baseline.